Aluminum and aluminum alloy metals are used in a variety of settings where they are subjected to wear and corrosion. After being cast or formed into desired shapes, aluminum surfaces usually receive protective coatings, such as layers of aluminum oxide, paint, or wax, to resist wear or corrosion.
Before protective coatings are applied, the metal is generally treated by degreasing, alkaline cleaning, rinsing, and deoxidizing the surface.
A deoxidized aluminum surface may be anodized to convert surface aluminum metal to corrosion resistant aluminum oxide. Alternatively, after deoxidation, an aluminum surface may be treated to form an oxidation or layer containing metal salts on the aluminum surface. Two commonly used solutions for surface protection are marketed under the trademarks ALODINE and IRIDITE. Each solution contains chromic acid. After anodizing or conversion coating has been completed, a paint, wax, or other additional protective coatings may be applied.
Corrosion resistance of aluminum is dependent, in part, on successfully deoxidizing of the surface prior to conversion coating or anodizing. Successful deoxidizing requires a thoroughly clean surface free from grease, coatings, or other impurities. In addition, the surface should be activated to homogenize the receptivity of the surface to subsequent protective coatings.
Deoxidation usually is achieved with chromated acid solutions, but these solutions are ineffective at removing smut, caused by caustic solutions. In particular, chromated deoxidizers have difficulty in removing smut on cast alloys or weld zones on aluminum weldments. Brushing has ordinarily been necessary to remove smut from the aluminum after deoxidizing.
Chromated acid deoxidizing solutions are environmentally undesirable and the Environmental Protection Agency (EPA) has enacted regulations which restrict chromium effluents. Consequently, in more and more metal finishing facilities, chromium treatment plants are being installed at costs of approximately $350,000. Annual operating costs of each plant is presently about $30,000/year. Still, restrictions on solid chromium disposal make its handling expensive.
Nonchromated deoxidizing solutions are known, but water contamination usually causes these nonchromated deoxidizers to lose quickly their ability to desmut aluminum alloy metals. One nonchromated deoxidizing solution is described in U.S. Pat. No. 4,346,128, and includes a mixture of 50 vol % HNO.sub.3 (40.degree.-42.degree. Baume, O-N-350), 25 vol % H.sub.2 SO.sub.4 (66.degree. Baume, O-S-809), 25 vol % H.sub.2 O, and 1 lb/gal ammoniumfluoride (usually ACTANE 70 from Enthone, Inc.). This solution has an etch rate of about 0.9 mil/surface-hr on A357 cast aluminum, and can remove smut from aluminum.
Enthone, Inc. promotes mixtures of nitric acid, sulfuric acid and ammoniumbifluoride for etching glass and pickling titanium.